(Bacillariophyta): a Description of a New Araphid Diatom Genus Based on Observations of Frustule and Auxospore Structure and 18S Rdna Phylogeny
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Phycologia (2008) Volume 47 (4), 371–391 Published 3 July 2008 Pseudostriatella (Bacillariophyta): a description of a new araphid diatom genus based on observations of frustule and auxospore structure and 18S rDNA phylogeny 1 2 3 1 SHINYA SATO *, DAVID G. MANN ,SATOKO MATSUMOTO AND LINDA K. MEDLIN 1Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany 2Royal Botanic Garden, Edinburgh EH3 5LR, Scotland, United Kingdom 3Choshi Fisheries High School, 1-1-12 Nagatsuka Cho, Choshi City, Chiba, Japan S. SATO, D.G. MANN,S.MATSUMOTO AND L.K. MEDLIN. 2008. Pseudostriatella (Bacillariophyta): a description of a new araphid diatom genus based on observations of frustule and auxospore structure and 18S rDNA phylogeny. Phycologia 47: 371–391. DOI: 10.2216/08-02.1 Pseudostriatella oceanica gen et. sp. nov. is a marine benthic diatom that resembles Striatella unipunctata in gross morphology, attachment to the substratum by a mucilaginous stalk and possession of septate girdle bands. In light microscopy, P. oceanica can be distinguished from S. unipunctata by plastid shape, absence of truncation of the corners of the frustule, indiscernible striation and absence of polar rimoportulae. With scanning electron microscopy, P. oceanica can be distinguished by a prominent but unthickened longitudinal hyaline area, pegged areolae, multiple marginal rimoportulae and perforated septum. The hyaline area differs from the sterna of most pennate diatoms in being porous toward its expanded ends; in this respect, it resembles the elongate annuli of some centric diatoms, such as Attheya and Odontella. 18S rDNA phylogeny places P. oceanica among the pennate diatoms and supports a close relationship between P. oceanica and S. unipunctata, but the genetic distance between them, coupled with the morphological differences, justifies separation at genus level. However, the affinity of the P. oceanica – S. unipunctata clade remains unresolved both in molecular and in morphological study. Both genera are only distantly related to Hyalosira and Grammatophora, despite similarities in frustule structure and growth habit, arguing against their inclusion in the same family. The auxospore is covered with series of transverse and longitudinal bands, but the structure and arrangement of these bands appear to be more similar to the properizonia of some centric diatoms than to the classic type of perizonium seen in other pennate diatoms; a few scales are also present. The differences between properizonia and perizonia are discussed. KEY WORDS: 18S rDNA, Araphid diatom, Auxospore, Evolution, Fine structure, Morphology, Perizonium, Phylogeny, Pseudostriatella oceanica, Striatella, Taxonomy INTRODUCTION high abundance, the defining features of the main groups of araphid diatoms are not fully established. Benthic diatoms are ubiquitous in shallow coastal environ- To obtain a more complete picture of the natural history ments and are one of the most taxonomically diverse of araphid diatoms, we have been collecting samples groups of organisms in estuarine ecosystems (Sullivan & worldwide from coastal regions. Recently we encountered Currin 2000). Because of their high primary production a new diatom that superficially resembled Striatella rates, benthic diatoms play an important role in the unipunctata (Lyngbye) Agardh. Scanning electron micros- functioning of benthic trophic webs in intertidal mudflats copy (SEM) revealed, however, that this diatom differed and shallow-water ecosystems of temperate to tropical from S. unipunctata in several features that are generally regions (Cahoon 1999; Underwood & Kromkamp 1999). used as taxonomic characters among araphid diatoms, Araphid pennate diatoms (diatoms with a sternum but including characteristics of the sternum, striae, areolae, lacking a raphe system; see Terminology) are important apical pore field, rimoportula and septum. Given these components of these coastal assemblages, particularly observations, together with information on the plastids and among communities attached to macrophytes and macro- 18S rDNA sequences, we conclude that the diatom should algae, animals, rocks and sand grains (Round et al. 1990). be described as a new genus, Pseudostriatella. Taxonomically, araphid diatoms have long been neglected, We have also been able to make detailed observations on perhaps because of their morphological simplicity; accord- the fine structure of auxospores produced spontaneously in ing to Round et al. (1990), ‘in many ways the classification monoclonal cultures. With the advent of electron micros- of the araphid group is the most difficult because unlike the copy, particularly SEM, information about auxospore centric series their valve structure is rather simple, and structure has greatly increased (e.g. Crawford 1974; Mann unlike the raphid series, the plastids and their arrangements 1982b; von Stosch 1982; Cohn et al. 1989; Kaczmarska et have few distinguishing features’. Thus, in spite of their al. 2000, 2001; Kobayashi et al. 2001; Schmid & Crawford 2001; Nagumo 2003; Sato et al. 2004, 2008a, b; Amato et al. * Corresponding author ([email protected]). 2005; Tiffany 2005; Toyoda et al. 2005, 2006; Trobajo et al. 371 372 Phycologia, Vol. 47 (4), 2008 2006; Poulı´cˇkova´ & Mann 2006; Poulı´cˇkova´ et al. 2007). was left at room temperature for c. 30 min; and (5) steps 1 However, although it has become clear that some aspects of and 2 were repeated several times to remove decomposition the fine structure of auxospores have phylogenetic signif- products. Cleaned frustules were then mounted in Mount- icance (e.g. Medlin & Kazcmarska 2004), there is still media (refractive index n20/D 5 1.50; Wako). insufficient information to reveal how the structure and For SEM examination, cleaned material was air-dried development of auxospores have evolved in the major onto coverslips. To observe auxospores, coverslips to which diatom groups, especially among the lineages of araphid the auxospore mother cells had already become attached pennate diatoms. Indeed, the only detailed information were immersed in 10% glutaraldehyde for 1 h at room available concerning araphid pennates is the account of temperature, then washed with distilled water, air-dried and Rhabdonema Ku¨tzing by von Stosch (1962, 1982) and the fixed to SEM stubs with carbon tape. For observations of SEM studies of Gephyria media Arnott (Sato et al. 2004), cells still attached to the substratum by mucilaginous stalks, Grammatophora marina (Lyngbye) Ku¨tzing (Sato et al. host plants were fixed with 10% glutaraldehyde for 2 h at 2008a) and Tabularia parva (Ku¨tzing) Williams & Round 4uC, rinsed with distilled water several times to remove the (Sato et al. 2008b). In the present study, we compare the glutaraldehyde, dehydrated using increasing concentrations auxospore fine structure in these diatoms with that of of t-butyl alcohol and freeze-dried using a JFD-310 Pseudostriatella oceanica and discuss the evolutionary instrument (JEOL). Freeze-dried specimens were attached relationships of Pseudostriatella. to the stub directly with carbon tape. All SEM specimens were coated with gold using an SC 500 sputter coater (Emscope). A QUANTA 200F (FEI) was used for SEM MATERIAL AND METHODS observation at an accelerating voltage of 3–10 kV and c. 10 mm working distance. All the images included in this paper are from cultured strains, except for those from Collections and cultures freeze-dried material (Figs 9–13). Captured images were Both natural specimens and clonal cultures were examined adjusted with Adobe Photoshop. in this study. Vegetative cells of the P. oceanica examined here were collected by S. Matsumoto at Yumigahama DNA methods Beach, Minamiizu, Shizuoka Prefecture, Japan, on 20 May c. 2005, attached to Cladophora sp., and by B.K. Petkus at Samples of 500 ml of culture were filtered through 3-mm- Horseneck State Beach, Westport, Massachusetts, USA, on pore-diameter membrane filters (Millipore). Filters were August 2006, from bottom sand. For morphological immersed in 500 ml DNA extraction buffer containing 2% comparison, S. unipunctata, the generitype of the genus (w/v) CTAB, 1.4 M NaCl, 20 mM EDTA, 100 mM Tris- Striatella, was collected by L.K. Medlin from Banyuls sur HCl, pH 8, 0.2% (w/v) PVP, 0.01% (w/v) SDS and 0.2% b- Mer, France, on 13 February 2005. Single cells were mercaptoethanol. Immersed filters were incubated at 65uC isolated from the American and French samples to obtain for 5 min, vortexed for a few seconds and then discarded. clonal cultures. Cultures were maintained in IMR medium Subsequently, the buffer was cooled briefly on ice. DNA was extracted with an equal volume of chloroform–isoamyl (Eppley et al. 1967) at 15uC under cool-white fluorescent light on a 14 : 10-h (L : D) photoperiod at a photon flux alcohol (24 : 1 [v/v]) and centrifuged in a tabletop density of 30–40 mmol photons m22 s21. A coverslip was Eppendorf microfuge (Eppendorf) at maximum speed placed on the bottom of the culture vessel to be colonized (14,000 rpm) for 10 min. The aqueous phase was collected, with cells producing auxospores. Both strains examined in re-extracted with chloroform–isoamyl alcohol and centri- this study, P. oceanica s0384 and S. unipunctata s0208, are fuged as described previously. Next, the aqueous phase was currently available on request to the first author but may mixed thoroughly with 0.8 volumes of ice-cold 100% not survive long-term in culture (cf. Chepurnov et al. 2004). isopropanol, left on ice for 5